In a wireless communication system, especially in a mobile communication system, fading occurs from times to times. Buildings, mountains, and foliage on the transmission path between a transmitter and a receiver can cause reflection, diffraction, and scattering on a propagating electromagnetic wave. The electromagnetic waves reflected from various large objects, travel along different paths of varying lengths. If there is an obstacle with sharp irregularities on the transmission path, the secondary waves resulting from the obstructing surface are present around the obstacle. Also if there are small objects, rough surfaces, and other irregularities on the transmission path, scattered waves are created. All these waves will interact with each other and result in multipath fading.
Usually there are two methods to deal with the multipath fading. One method is to use a multipath signal combiner such as a RAKE receiver to combine all the significant paths of a multipath-fading signal together. Another method is to use an equalizer to eliminate all the paths of a multipath-fading signal except the strongest path. Under some scenarios, however, neither signal combiner alone nor decision feedback equalizer alone works well.
There is a corresponding component signal for each path of a multipath-fading signal. A multipath signal combiner in a receiver is to combine all the significant component signals according to their corresponding signal strengths. It works effectively when the most significant component signals have almost same strength and these component signals do not nullify each other. On average, a multipath signal combiner can provide a signal more stable and stronger than each individual component signal. For a multipath radio link with multipath fading, a transmitted symbol appears several times at a receiver in the time domain with each one corresponding to a different path. When transmission rate is low, the multipath fading spans in less than one symbol period. Though the multipath component signals interfere with each other, the transmitted symbols do not cancel each other totally. But as the transmission rate is higher and higher, a multipath fading can span over one or more symbol periods and a symbol on one path could almost cancel a previous symbol on another path. When symbols almost cancel each other or possess nulls, a multipath combiner has nothing to combine and therefore system performance declines.
A decision-feedback equalizer in a receiver, on another hand, is effective to compensate the nulls mentioned above. Basically, an equalizer in a receiver is to keep the strongest path and eliminate all other paths. It works effectively when there is a strong and stable path. Under some environments such as in many metropolitan areas, there is no line-of-sight signal. The received signal is a multipath-fading signal from reflection, scattering, and diffraction. Statistically no any particular component signal of the multipath-fading signal is stable for a relatively long period of time and significant stronger than the rest component signals in a fairly large region. Another problem with regular decision feedback equalizer is that the feedforward filter may not work efficiently when there is no strong line-of-sight signal. Since future symbols are under various intersymbol interference and noise, in order to make sure convergence, the coefficients of the feedforward filter for getting rid of the interference from future symbols to current symbol have to be relatively small.
Therefore when a multipath fading spans one or more symbol periods and no any path is stable and strong over a relatively large area for a relatively long period of time, either multipath combiner alone or equalizer alone does not work effectively. One example for this kind of scenario is a high-speed mobile receiver in metropolitan area. In order to have reliable communication under this kind of scenario, one may want to use both coherent signal combiner and decision feedback equalizer with a better estimation of the interference of future and past symbols to current symbol. Also a plurality of decision feedback equalizers with embedded coherent signal combiners can cascade one by one to further improve the system performance. With multistage structure, an equalizer at a later stage can provide a better estimation of the interference of future and past symbols to current symbol then one in an earlier stage.